1. Field
The disclosed subject matter of the present application relates to a surface mounting semiconductor device, and more particularly to a surface mounting semiconductor device that can be equipped with a semiconductor element such as an IC, a diode, a light-emitting diode (LED), a photodiode, a PIN diode, a phototransistor, and/or other similar semiconductor components.
2. Description of the Related Art
Surface mounting semiconductor devices with various structures have been previously proposed. One conventional art example is structured as shown in FIG. 12 and includes lead frames 50a, 50b, 50c that are separated and arranged at three locations. On the first lead frame 50a, a semiconductor element (such as an LED chip 51) is mounted, with a thermal conductive adhesive interposed therebetween. Electrodes provided at two locations on the upper side of the LED chip 51 are connected through bonding wires 52b and 52c to the second lead frame 50b and the third lead frame 50c, respectively. Thus, the upper electrodes on the LED chip 51 are electrically connected to the second lead frame 50b and the third lead frame 50c, respectively.
The LED chip 51, the bonding wires 52b, 52c, and parts of the lead frames 50a, 50b, 50c are sealed in a transparent resin 53. In this case, as for the second lead frame 50b and the third lead frame 50c, one end of each is sealed in the transparent resin 53 and the other end of each protrudes from the transparent resin 53 to an external location. As for the first lead frame 50a, a lowermost surface 54aprotrudes from the transparent resin 53 to an external location. The second lead frame 50b and the third lead frame 50c have respective lowermost surfaces 54b, 54c defining portions protruding from the transparent resin 53, which are located almost on the same plane as the lowermost surface 54a of the first lead frame 50a. 
The surface mounting LED device (hereinafter referred to as “LED device”) thus structured may be mounted as follows. For example, conductor patterns 55a, 55b, 55c separately formed at three locations on a circuit board are bonded to the lowermost surfaces 54a, 54b, 54c of the lead frames of the LED device via solder 57 having electrical conduction and thermal conduction properties.
Thus, heat radiated from the LED chip 51 can be conducted through the first lead frame 50a and the solder 57 in the LED device to the conductive pattern 55a and the circuit board 56 and then released to the atmosphere. As a result, elevation of the temperature at the LED chip 51 can be prevented and the light emission efficiency (brightness) can be sustained. In addition, factor(s) promoting deterioration of the LED chip 51 can be reduced and a long lifetime (an improvement in reliability) can be achieved (see, for example, Japanese Patent Document No.: JP-A 2002-252373, the disclosure of which is hereby incorporated by reference, and an English translation of which is submitted on even date with this application via an Information Disclosure Statement).
When the above-structured LED device is mounted on the circuit board, the following malfunction may arise. As shown in FIG. 13, the second lead frame 50b and the third lead frame 50c protrude from the transparent resin 53 of the LED device and have solder joints with the conductor patterns 55b, 55c on the circuit board 56. These solder joints are different in terms of condition from the solder joint between the first lead frame 50a equipped with the LED chip 51 and the conductor pattern 55a. Therefore, the LED device may be mounted in such a manner that the optical axis X of the LED device tilts α° toward a reference line normal to the circuit board 56 (the LED device tilts α° toward the circuit board 56).
Malfunction(s) may be caused by the following factor(s). As for the solder interposed between a solder joint surface (outer, exposed surface) of the first lead frame 50a equipped with the LED chip 51 and a solder joint surface of the conductor pattern 55a, the wetness and spread (e.g., viscosity) of solder is suppressed. Accordingly, solder stays in the space between the solder joint surface of the lead frame 50a and the solder joint surface of the conductor pattern 55a, resulting in formation of a certain thickness of solder between both joint surfaces.
The respective solder joint surfaces of the second lead frame 50b and the third lead frame 50c and the respective solder joint surfaces of the conductor patterns 55b, 55c have joints therebetween. In the joints, the solder between both joint surfaces may partly shift to a particular side to form solder fillets 58 at the time of jointing. Therefore, the thickness of solder between both joint surfaces is made thinner than the thickness of printed solder by the extent of the solder shifted to the side. In this case, the respective solder joint surfaces of the second and third lead frames 50b, 50c and the respective solder joint surfaces of the conductor patterns 55b, 55c have a thickness of solder therebetween. This thickness of solder is made thinner than the certain thickness of solder between the solder joint surface of the lead frame 50a and the solder joint surface of the conductor pattern 55a. 
Therefore, when the certain thickness of solder is formed between the solder joint surface of the lead frame 50a and the solder joint surface of the conductor pattern 55a, this certain thickness of solder exerts an influence. In this case, either the second lead frame 50b or the third lead frame 50c lifts and tilts the LED device toward the circuit board 56.
In particular, when the lowermost surface of the first lead frame 50a equipped with the LED chip 51 is located almost on the same plane as the lowermost surfaces of the second lead frame 50b and the third lead frame 50c which is arranged at both sides thereof, the tilt of the mounted LED device toward the circuit board 56 becomes more remarkable.
Therefore, the conditions of formation of the solder joints are not made uniform as described above, and accordingly the LED device is mounted in a poor state and made unstable, with poor reliability on the solder joint.
Originally, when part of the first lead frame 50a equipped with the LED chip 51 is externally exposed, an effect of thermal radiation can be expected. The lack of reliability on the solder joint due to the poor mounting of the LED device lowers the conduction of heat to the circuit board 56 and the conductor pattern 55a formed on the circuit board 56, and prevents sufficient thermal radiation.
Particularly, in an LED device of the type that requires a large current to drive the LED chip 51, a failure to achieve excellent thermal radiation results in a large influence on reducing the efficiency of light emission.
The reproducibility of the tilt of the mounted LED device toward the circuit board 56 is poor with regard to the tilted angle α, resulting in worsened optical characteristics such as variations in distribution characteristic of the LED light.
An interval between two adjacent lead frames of the lead frames 50a, 50b, 50c is relatively short. The lead frames 50a, 50b, 50c are bonded to the conductor patterns 55a, 55b, 55c through the solder joints, respectively. Accordingly, the solder joints may be brought into contact with each other to cause electrical malfunctions.
After the LED device is mounted on the circuit board 56, the state of the joints between the lead frames 50a, 50b, 50c and the conductor patterns 55a, 55b, 55c can not necessarily be identified by image-recognition or by visual inspection. Thus, it may be impossible to eliminate occurrences of failed products within the production processes.